Project Summary The objective of this proposal is to determine how cells optimize survival in the face of adverse environmental conditions. The proposed research is a continuation of a long-standing program designed to understand how stress signaling pathways including phosphorylation of eIF2?, hypophosphorylation of 4EBP-1, and angiogenin-induced tRNA cleavage re-program protein translation and induce the assembly of stress granules (SGs) to modulate cell survival. Our central hypothesis is that these signaling pathways modulate the expression of pro-growth and pro-survival proteins in ways that can be exploited for the treatment of cancer and neurodegenerative disease. This is based upon our findings that ANG selectively cleaves tRNAs to produce bioactive fragments (i.e., tiRNAs) that use multiple mechanisms to inhibit translation initiation and induce SG assembly. We have also found that different stress stimuli produce compositionally distinct SGs that can either promote or inhibit cell survival. The rationale for the proposed research is that, once we know how these signaling pathways re-program protein translation to modulate cell survival, we will be able to exploit these events to treat cancer and neurodegenerative disease. We will test our central hypothesis by addressing key knowledge gaps that are holding back progress in the field. These include: 1) an understanding of the role played by RNA in the nucleation of SG assembly, 2) an understanding of the composition and function of SGs that either promote or inhibit cell survival, 3) an understanding of the mechanism by which the SG nucleating proteins G3BP1/2 promote SG assembly, 4) elucidation of the composition of tiRNA inhibitory complexes and the identify of their target transcripts, 5) an understanding of how tiRNAs selectively modulate the translation of mRNAs bearing 5'-terminal oligopyrimidine motifs, 6) an understanding of how G-quadruplex structures modulate tiRNA function, and 7) elucidation of the different mechanisms by which individual tiRNAs inhibit translation initiation. The contribution of the proposed research will be to determine how tiRNAs and SGs modulate the survival of cells involved in the pathogenesis of cancer and neurodegenerative disease. This contribution is significant because it provides a molecular basis for the development of pharmacologic strategies to prevent stress-mediated tumor growth or motor neuron death. The proposed research is innovative because it focuses on the downstream effectors of these stress response programs and attempts to identify molecular targets that can be developed into novel therapeutics.